1. Field of the Invention
The present invention relates to a channel scanning method and apparatus for use in a cellular telephone system, and more particular, to a method and apparatus for scanning control and access channels during a period of reduced battery consumption.
2. Discussion of Related Art
Cellular telephone systems are designed to permit radio communications with mobile stations which, by their mobility, can cross boundaries between radio zones, or cells, the cells being defined as areas serviced by relatively immobile base stations within the cells. Because of their need for mobility, the mobile stations typically are battery operated, whether vehicle mounted, portable, or pocket telephones. The battery life becomes an important factor in cellular systems and, consequently, power consumption is an important issue for mobile stations.
In order to increase battery life, various mechanisms for decreasing power consumption have evolved. One such mechanism is to impose one or more of an idle mode, a standby mode, and a sleep mode on mobile telephones wherein in these modes various functions and/or portions of the mobile telephone circuitry are turned off to decrease overall power consumption.
In the idle mode, a mobile telephone operating within, e.g., the Nordic Mobile Telephone (NMT) system is locked on a control channel (CC), sometimes referred to as a calling channel, to be ready to receive a page, which is the first step in initiating a telephone link with a base station. The mobile station is not actively engaged in a voice communications link, i.e., a conversation. Under the NMT standard, in the standby mode the mobile station rests on a calling channel with valid traffic area number and calling channel prefix. In the standby mode, the mobile station is ready for reception or initiation of a call. If the mobile station loses a lock on the calling channel (the signal strength falls below a given limit), it starts a scanning procedure for searching for a new, stronger calling channel. If the user initiates a call, the mobile station leaves the standby mode and enters the procedure for initiating a call according to NMT standards. In a sleep mode, the mobile station is responsive to the fewest functions, limited to only user initiated functions.
Conventionally, a mobile telephone does not continuously scan for the strongest control channel. When a call is placed, an NMT telephone randomly selects a starting channel among a group of prestored access channels (AC's), to begin an access channel scan. The mobile station will select and use the first access channel whose signal strength is sufficiently high. The selected access channel is then used to initiate a call link set-up procedure.
To save battery life, in the NMT system for example, base stations send out battery saving orders (BSO's) to groups of mobile stations. The groupings are designated by the mobile system. The system, by issuance of BSO's, orders groups of mobile stations to go into a sleep mode, wherein the greater part of the circuitry of the mobile station becomes inactive to further reduce battery consumption. A typical BSO period is a few seconds, and the length of the BSO period is usually determined by the call frequency in the system (e.g., the load at the mobile switching center which services a group of base stations). If a call is placed to a mobile station during the period designated by the BSO, the call will be put on hold in the mobile switching center (which controls several base stations) until the BSO period has terminated.